Description:
As DROP USER lowercases the host name part of the user name now (see MySQL bug http://bugs.mysql.com/36742) it is not possible to drop users with host names with upper case letters in them (either from pre-5.1.53 installations or due to bug #1002404)

Such users either need to be removed by either removing them from the mysql.user table using DELETE or by first converting the host name part in mysql.user to all lower case using UPDATE and LOWER

(See also MySQL bug http://bugs.mysql.com/61525)

How to repeat:
(assuming that bug #1002404 has not been fixed yet)

on the shell do

  hostname FOOBAR

in the mysql command line client

  SELECT User, Hostname FROM mysql.user WHERE Host = 'FOOBAR';

  DROP USER 'root'@'FOOBAR';

  SELECT User, Hostname FROM mysql.user WHERE Host = 'FOOBAR';

the DROP will report "0 rows affected" and both SELECTs will return the same result, proving that no user was deleted by the DROP statement

Suggested fix:
Make DROP USER really case insensitive regarding to the host name part